import java.util.Scanner;

// Press Shift twice to open the Search Everywhere dialog and type `show whitespaces`,
// then press Enter. You can now see whitespace characters in your code.
public class Main {
    public static void main(String[] args) {

        Scanner reader = new Scanner(System.in);
        double x=reader.nextDouble(),y=reader.nextDouble(),z=reader.nextDouble();

        double sum = (x+y*2-z)/2;
        System.out.println("sum="+sum);
    }
}